In a known safety switch mechanism which is adapted to be fitted to an enclosure having a door, gate or protective cover, the switch assembly is adapted to switch OFF an electrical power supply when the door, gate or protective cover is opened. The known safety switch mechanism comprises a safety switch adapted to be fitted to the enclosure and an actuator adapted to be fitted to the door, gate or protective cover, and insertable into the safety switch to turn ON the electrical power when the enclosure is closed by the door, gate or protective cover.
The safety switch comprises within a housing, at least one contact set comprising at least one fixed contact, and at least one moveable contact which is carried by an axially moveable push rod spring loaded to maintain the sets of contacts apart and consequently the power supply OFF.
The axially moveable push rod is connected to a roller cam mechanism that is mounted rotatably, usually in a housing, and adapted to be rotated by the actuator when inserted through an aperture in the housing. In one known construction, the roller cam mechanism comprises a pair of roller portions axially spaced and rotatably supported on a shaft. The push rod has a cam follower pin engageable in a cam slot in each of the roller portions. Rotation of the roller cam mechanism causes the axially moveable push rod to be pushed axially to make the contacts and turn on electrical power.
The pinned connection operates in conjunction with the spring biasing to pull off the contacts when the roller cam is rotated on removal of the actuator.
The presence of the physical inter-connection of the roller cam and the axial push rod is necessary with the push on-pull off construction, but can be disadvantageous in certain failure situations. Furthermore, the fact that the contacts are pushed on and/or pulled off can give rise to problems with electronic monitoring of contact condition especially where there are multiple contact sets carried in axially spaced relation by the axially moveable push rod.
Electronic monitoring of contact condition is being increasingly employed to augment the physical safety precautions provided by such safety switches. The electronic monitoring of contact condition, serves as a double check as well as providing a means of accounting for mechanical failure of the switch mechanism. Thus, for example, the monitoring may require all contacts to make and/or break within a specified period. Ideally the period should be as short as possible to minimise down time, and to ensure a fast response time to shut down the machinery for safety purposes. It is an objective of the present invention to provide an improved switch mechanism.